For the same kite and board kite size is pretty much proportional to weight from pure physics point of view as I worked out here in a thread in September:
http://kiteforum.com/viewtopic.php?f=1 ... r#p1027094
So if your 7,9, 12 are real numbers people use on your beach you would need to multiply by 100/77 = 1.298 = 1.3
If you use a bigger board by about 5% surface area (and the same make) than the 77 kg person you will need 1.3/1.05= 1.24 times bigger kite.
So assuming you do get 5% bigger board theoretical values are:
7 : 8.68
If you use 10% bigger board i.e. say 44 cm wide where 77 kg person uses 40 cm (the same lenght and make obviously),
then you can use 5% smaller kites again.
This is very rough but gives you an idea for any weight.
Obviously it only works if you know that 7,9, 12 are good for 77 kg rider at the beach in question - i.e.wind range on the beach is good.
Hope this helps
NOTE I am a beginner with an engineering approach to life (not kite expert in any sense)